This invention relates to coatings, films and sheets including non-spherical particles. More particularly, it relates to a coated article and method for orienting such particles with respect to an article surface.
Coatings such as films and paints, as well as sheets of material, frequently are used to provide an artistic effect on a surface of an article. For control of brightness of or reflection from a surface, coatings and sheets have used non-spherical metallic particles in the shape of flakes having a major dimension, with the relative orientation of the flake and the major dimensions in respect to the article surface determining the degree of brightness or reflection. In addition, bright films and sheets are useful because of certain physical properties such as that of reducing emission of heat from a surface. Such coatings have been applied to components of power generating apparatus, for example turbine engine components, as well as to components of vehicles, for example surface members of airplanes, boats, automobiles, etc.
A number of methods for using a magnetic field for controlling the orientation of non-spherical particles, such as metallic flakes, in a coating on an article have been reported. Other reported methods employ an ion effect or corona for flake positioning.
In U.S. Pat. No. 2,418,479xe2x80x94Pratt et al. (patented Apr. 8, 1947), metallic flake pigments, such as ferromagnetic flakes, in paint films are positioned on a simple, planar surface by reaction to a magnetic field. Both the article surface and the flakes are located in the direction of the magnetic field. The flakes rotate as a result of a torque force from the magnetic field. This method requires that the article surface on which the film is disposed lie between magnetic poles so that each long or major dimension of the particles will align itself along the magnetic field direction, as does the needle of a compass. Use of this method is impractical for large surfaces of articles since the magnetic field strength would have to be extremely large and costly to construct. In addition, such method as described would not operate to orient a majority of the flakes disposed in a film or coating on a curved or complex shaped, non-planar article surface. One example of such a surface is an annular or airfoil shaped component of power generating apparatus such as a gas turbine engine.
Another method using a magnetic field to orient or de-orient such non-spherical particles at localized surface areas of an article is reported in U.S. Pat. No. 5,630,877xe2x80x94Kashiwagi et al. (patented May 20, 1997), in order to produce visually discernible patterns. That method impresses a desired pattern on an article surface of the particles in various different positions of orientation with respect to the surface using a shaped magnet held in a fixed position to the surface. When the magnetic field is applied at the fixed position, the orientation of the particles at the various angles to the article surface is determined by location of each particle in the fixed magnetic field and the relative strength and torque of the fixed magnetic field on the particles. The magnet must remain in a fixed position because any movement of the magnetic field along the article surface would destroy the desired pattern of the particles at the different orientations.
Methods reported in such patents as U.S. Pat. Nos. 4,818,627 and 4,911,947xe2x80x94Melcher et al. (patented Apr. 4, 1989 and Mar. 27, 1990, respectively) subject metallic particles on a film on an electrically conductive substrate to a corona or ion current. This orients the particles substantially in the direction of the current, generally substantially perpendicular to the article surface.
The present invention provides an article, in one form including a non-planar article surface coated with non-spherical particles, and in another form as a sheet of material including a sheet or article surface substantially along the plane of the sheet. Each particle includes a major dimension with an average of at least about 50% of the particles having the major dimension oriented generally along the article surface in respect to which each particle is disposed.
In another form, the present invention provides a method for orienting with respect to an article surface a plurality of non-spherical particles each including a major dimension and each of which can be moved by a force on the particles. The particles are disposed in respect to the article surface in a fluid medium the viscosity of which can be increased to secure the particles in a position.
In one form of such method, substantially parallel relative movement between a magnetic field, and each particle and the article surface in respect to which each particle is disposed, is provided. The magnetic field is disposed with its direction relative to the particles and the article surface so that, during the relative movement, the magnetic field will locate an average of at least about 50% of the major dimensions in a position generally along the article surface in respect to which each particle is disposed. This relative movement between the particles and the magnetic field moves the particles by a torque force from the magnetic field to their respective position. After particle positioning, the viscosity of the medium is increased to secure the particles in the positions.
In another form of such method, the force on the particles to orient the particles generally along the article surface is provided by inducing flow in a medium carrying the particles. Such force on the medium applies a force to turn the particles in the direction of flow. Such medium flow disposes the particles each with their major dimension generally along to the article surface. Resulting from this form of the method are several forms of articles: one is an article including a coating on an article surface; another is a sheet of material. The non-spherical particles are oriented with the major dimension oriented generally along either the article surface on which the coating is disposed or the plane of the sheet that includes the particles, as the article surface.
In still another form of such method, the force on the particles to orient a majority of the major dimensions of the particles generally along the article surface is the force of gravity. Gravity is allowed to act on the particles while the medium is maintained in a fluid state for a time sufficient to enable a majority of the major dimensions of the particles to become so oriented.
In a further form of the present invention, the force on the particles to provide such orientation is the surface tension of the fluid medium that is selected to provide such a force.